A Joint Analysis of Groundwater Changes and Crustal Deformation Related to Earthquake Swarms off the East

Coast of Izu Peninsula, Japan

○N. Koizumi and S.Itaba (AIST, Active Fault and Earthquake Research Center,

Geological Survey of Japan)

OUTLINE １・Earthquake Swarms off the East Coast of Izu Peninsula, Japan. They Repeatedly Caused Crustal Deformation and Groundwater Changes.

2・Different Fault Models for Explaining Individual Crustal Deformation and Groundwater Level Change for the Earthquake Swarms.

３．A Joint Analysis of Groundwater Changes and Crustal Deformation for a Comprehensive Model .

４．Examination of the Comprehensive Model.

Ito City

Crustal Deformation (GPS,Tilt,Vol.Strain)

Groundwater Level

GPS

Tilt

Vol.Strain

10km Brittle

Ductile

Ito City

Crustal Deformation (GPS,Tilt,Vol.Strain)

Groundwater Level

GPS

Tilt

Vol.Strain

DEPTH<10km

10km Ductile

Brittle

Preseismic Change in GWL & Crustal Deformation.

Prompt Grasp of the Depth of the Magma Head is More Important.

1978-1989 1990-2000

▲

2001-

Since 1990

Direction NW-SE or WNW-ESE (=Direction of PSP Motion)

Swarms are Approaching Ito City, Which has a Population of 70,000.

History of the Swarms

Ito City

Dec.17–21,2009 5km

(JMA,2010)

17 18 19 20 21

Dec. 2009

1day

5

15

10

20

0 km

Temporal Change in Hypocenters in the Rectangle

Ito City

5km

OMR

HGS

OMR

HGS

5km

OMR

HGS

HGS

OMR

OMR

-0.1

-0.2

-0.3

-0.4

-0.5

0ppm

V.S., V.S-GWL

5km

Tilt Change

(NIED,2010)

Earthquakes (count/h)

Atmospheric P.

Rainfall (mm/h)

IT2H

YOSH

OKAH

TNGH

Tilt Change

(NIED,2010)

Earthquakes (count/h)

Atmospheric P.

Rainfall (mm/h)

IT2H

YOSH

OKAH

TNGH

5km

2000nrad.

5000nrad. d.

5km 5km

GPS (7 Stations) (2009/12/9-12/15) - (2009/12/21-12/27)

Horizontal Vertical 3cm 3cm

Ito City

Crustal Deformation (GPS,Tilt,Vol.Strain)

Groundwater Level

GPS

Tilt

Vol.Strain

DEPTH<10km

W:(km)

O:(cm)

D (km)→Depth of the Magma Head

Dip

Start End OBS. F.Type Strike (Deg.)

Dip (Deg.)

D (km) L (km)

W (km)

O (cm)

Vol. 1E6m3

JMA１ 12/16 12/18 V.STRAIN

Tensile 120 91 3.5 1.3 3.2 53 2.2

JMA２ 12/18 12/20 Tensile 135 90 3.0 2.1 0.5 220 2.3

NIED1 12/17 12/18 Tilt

Tensile 127 108 1.9 3.0 5.3 31 4.9

NIED2 12/18 12/21 Tensile 129 90 2.4 0.6 1.1 210 1.4

GSJ, AIST

12/16 12/20 W,L V.Strain

Tensile 110 90 1.0 4.3 3.0 100 12.9

GSI1 12/9-12/15

12/21-12/27

GPS and Other Geodetic Measureme

Tensile 122 95 2.4 1.0 5.5 66 3.6

17 18 19 20 21

1day

5

15

10

20

0 km

2-3km

The Comprehensive Model is Needed.

5km

HGS OMR

-0.1

-0.2

-0.3

-0.4

-0.5

0ppm

GPS(H)

V.S., V.S.-GWL

3cm

GPS(V)

3cm

IT2H

YOSH

OKAH

TNGH

Tilt

5000nrad

HGS

OMR

OMR OMR

HGS HGS

1.Method of Weighted Least Squares

ΣWiRi2→Minimum Ri=(Observed Value – Calculated Value) Wi:Weight = 1/σi２ σi:Noise Level→WiRi2 :Non-Dim.

・Each Noise Level . (n: 1E-9), V.Strain:22nstrain, V.Strain from GWL :88 nstrain Tilts:43-355 nrad GPS-H: 1cm GPS-V: 2.5cm (Temporary)

2.Grid Search : Initial Position and Shape←Hypocenter Depth[km] : 2 - 12, Δ：2 → 1 Dip[deg] : 60 - 100, 5 → 1 Strike[deg]: 90(270) - 130(310), 5 → 1 Lon[deg] : 139.1 - 139.18, 0.02 → 0.01 Lat[deg] : 34.92 - 35.0, 0.02 → 0.01 Length[km] : 1 - 8 , 2 → 1 Width[km] : 2 - 8 , 2 → 1 Opening[cm]: 0 - 100, 10 → 0.5

A Joint Analysis for the Comprehensive Model

Start End OBS. F.Type Strike (Deg.)

Dip (Deg.)

D (km) L (km)

W (km)

O (cm)

Vol. 1E6m3

JMA１ 12/16 12/18 V.STRAIN

Tensile 120 91 3.5 1.3 3.2 53 2.2

JMA２ 12/18 12/20 Tensile 135 90 3.0 2.1 0.5 220 2.3

NIED1 12/17 12/18 Tilt

Tensile 127 108 1.9 3.0 5.3 31 4.9

NIED2 12/18 12/21 Tensile 129 90 2.4 0.6 1.1 210 1.4

GSJ, AIST

12/16 12/20 W,L V.Strain

Tensile 110 90 1.0 4.3 3.0 100 12.9

GSI1 12/9-12/15

12/21-12/27

GPS and Other Geodetic Measureme

Tensile 122 95 2.4 1.0 5.5 66 3.6

Com-Model

12/17 12/21

Tilt GWL: V.Strain、GPS

Tensile 127 92 1.0 4.0 4.0 10 1.5

Start End OBS. F.Type Strike (Deg.)

Dip (Deg.)

D (km) L (km)

W (km)

O (cm)

Vol. 1E6m3

JMA１ 12/16 12/18 V.STRAIN

Tensile 120 91 3.5 1.3 3.2 53 2.2

JMA２ 12/18 12/20 Tensile 135 90 3.0 2.1 0.5 220 2.3

NIED1 12/17 12/18 Tilt

Tensile 127 108 1.9 3.0 5.3 31 4.9

NIED2 12/18 12/21 Tensile 129 90 2.4 0.6 1.1 210 1.4

GSJ, AIST

12/16 12/20 W,L V.Strain

Tensile 110 90 1.0 4.3 3.0 100 12.9

GSI1 12/9-12/15

12/21-12/27

GPS and Other Geodetic Measureme

Tensile 122 95 2.4 1.0 5.5 66 3.6

Com-Model

12/17 12/21

Tilt GWL: V.Strain、GPS

Tensile 93 72 0.3 2.0 7.0 22 3.1

Depth (km) 0 2 4 6 8 10 km

Depth

(km

)

10

8

6

4

2

0

Comparison of Cal.V with Obs.V. 3cm 3cm

O C

O C

GPS-H GPS-V

TILT

5000nrad O C

-0.1

0ppm

-0.2

-0.3

-0.4

-0.5

HGS OMR

V.S., V.S.-GWL

O

C

O C

Matching of GPS Data is Worse Because of S/N is Worse.

Conclusion ・The Earthquake Swarms Have Repeatedly Occurred off the East Coast of Izu Peninsula Since 1978. They Have Been Approaching Ito City. The Swarms are Caused by the Magmatic Dike Intrusion, Which Induced Crustal Deformation and Groundwater Change. If the Head of the Magma Reach the Surface, Which Means a Volcanic Eruption, Severe Damage will Occur at Ito City. So It Is Important to Grasp the Position of the Magma Head by a Comprehensive Model Explaining All the Data. We Suggested one Method to Make the Comprehensive Model. ・In the Case of the Swarms in Dec.2009, Our Comprehensive Model Shows the Depth of the Magma Head was 0.3km, Which was Much Shallower than that Inferred From the Seismicity. We will Examine the Assumption of the Model and Parameters and Try to Grasp the Position of the Magma Head More Precisely.